Add Progressive Web App (PWA) template files for web_demo

[ktmeaton]

• I have followed the instructions in the PR template

This PR adds the minimal files for converting the web_demo into a Progressive Web App (PWA).

I think this might be of interest for mobile developers, as both Android and iOS have guides for how to publish a Progressive Web App to their stores.

• Android: https://developers.google.com/codelabs/pwa-in-play
• iOS: https://blog.pwabuilder.com/posts/publish-your-pwa-to-the-ios-app-store/
• Desktop too: https://learn.microsoft.com/en-us/microsoft-edge/progressive-web-apps-chromium/how-to/microsoft-store

A PWA let's you run your application as a standalone, offline app. This will work in both local sites for testing (screenshot below) as well as sites hosted on github pages (ex. https://ktmeaton.github.io/egui_demo_app/ is a PWA that can be installed to your computer or phone).

• On desktop chrome, the option to install the app will appear in the search bar*.
• On android chrome, this can be found under Add to home screen -> Install.
• On iOS safari, it's under Share -> Add to home screen

* On desktop, the user might need to satisfy engagement criteria, before the option becomes available.

On desktop the app will then show up in the system tray alongside other programs. On mobile, it will appear in the application tray just like you had installed it from the Play Store/App Store.

I couldn't find any egui docs/examples about this topic. And I had to fuse several blogs and tutorials to get it to work. So I thought there's a chance other people might be interested. If this is redundant, or you're not interested, please feel free to close!

[ktmeaton]

I found a bit more info in the eframe_template README.me about PWA setup! At least this PR description can be a docs pointer for anyone else searching for the "PWA" keyword in the egui repos.